An image reader such as a facsimile, a copying machine, an image scanner or the like is provided with a illuminating device incorporating a rod-like or rod-shaped light guide illuminating linear light on a document surface and an image sensor with a lens array focusing light reflected from the document surface on a line image sensor.
The illuminating device incorporating the rod-like light guide is configured such that light emitting means such as an LED is provided on one end of the rod-like light guide, light from the light emitting means is introduced from the end face into the inside of the guide, light propagating inside the rod-like light guide is scattered by light scattering patterns formed on a side of the rod-like light guide and scattered light is illuminated on the document surface from the light-emitting surface of the rod-like light guide.
In general, the rod-like light guide is of angular rod shape. The light scattering patterns are formed on the side of the light guide such that white pigment is printed by silk screen printing (Patent Document 1).
A method of printing above-described white pigment to form the light scattering patterns requires a printing process which of increases the cost of the guide and causes dispersion of characteristics of the light scattering patterns. Then, Patent Document 2 has disclosed that a concavo-convex portion is formed in advance on a mold cavity for injection molding of the light guide to print the concavo-convex portion on a side of the rod-like light guide to form the light scattering patterns.
In Patent Document 2, triangular grooves, in particular, are selected as the concavo-convex portion, the axis (longitudinal direction) of the triangular grooves is caused to coincide with a width direction of the side of the light guide or to slant relative to the width direction by a predetermined angle.
Patent Document 1: Japanese Patent Laid-Open No. 08-163320
Patent Document 2: Japanese Patent Laid-Open No. 2001-242322
FIGS. 12(a)-(c) are graphs showing relationships on each color basis between displacement in a main-scanning direction and light intensity of a light guide formed such that the triangular grooves thereof as light scattering patterns are formed to coincide with the width direction of the side of the light guide according to a first embodiment of the Patent Document 1. FIG. 12(a) shows light intensities at a floating amount of a document of 0 mm, FIG. 12(b) shows light intensities at a floating amount of a document of 3 mm and FIG. 12(c) shows light intensity ratios at a floating amount of a document of 0 mm and 3 mm.
As can be seen from FIG. 12(a), the intensities of light from each LED chip of RGB substantially coincide with each other at a main-scanning displacement of 10 mm or more, the blue (B) is weaker in intensity than other colors G and R at a region close to the incident end face with respect to a main-scanning displacement of 10 mm and the closer to the end face, the weaker the light intensity of each color becomes.
On the other hand, FIG. 12(b) shows that the light intensities of the blue (B) and the red (R) are extraordinarily high in the vicinity of a main-scanning displacement of 115 mm and the light intensity of the green (G) is higher than those of the blue (B) and the red (R) at a region closer to the incident end face with respect to a main-scanning displacement of 10 mm. In addition, as can be seen from FIG. 12(c), an extraordinary rise is observed on the blue (B) and the red (R) in the vicinity of a main-scanning displacement of 15 mm.
Thus, the extraordinary light intensity of specific colors, for example, the blue (B) and the red (R) in the vicinity of a main-scanning displacement of 15 mm emphasizes the blue (B) and the red (R) in the vicinity of a main-scanning displacement of 15 mm, resulting in unevenness in color.